chg: [opendata export] Parsing portal url parameter + slight parameters parsing changes
chg: [opendata export] Parsing portal url parameter + slight parameters parsing changes

- As the possibility of specifying the url of the
  Open data portal to use instead of the default
  one, we support here this parameter and adapt
  the way we build the command that will launch
  the python script
- Slight changes to replace some isset tests by
  empty tests to make sure the concerned fields
  are not only set, but also contain a value

MISP/MISP,MISP/MISP,MISP/MISP,MISP/MISP,SteveClement/MISP,SteveClement/MISP,SteveClement/MISP,SteveClement/MISP,MISP/MISP,SteveClement/MISP